Transformer & Generator selection for motor loads

The sizing depends on the inertia to be accelerated by the motor. If it is a pump or fan then 133 to 150% KVA compared to KW is OK. For crushers and conveyors 200% is suggested. This assumes that the voltage regulation on the HV side of the transformer is not excessive on start. Remember a soft starter is a soft mechanical start but still draws a lot of KVA as the squirrel cage induction motor has a power factor of only 0.3 on start up so it takes a lot of KVA at 0.3 to produce the KW required to accelerate the load.

Any large supplier such as Cat, Cummins, etc., has free software available to size generator sets The software depicts the voltage dip, engine power (speed droop) based on inputs of motor code, soft start, altitude, ambient, humidity, etc. If this is a large machine, such as 2-3 MW, then you need to get scientific with this and get into the reactance available of the generator. Remember that active power (watts) affects engine power availability and recovery ability (the governor, speed, fuel type, turbocharger curves, etc) and reactive power (VARs) affects the load of the generator [alternator] (heat via magnetism). The engine doesn't 'feel' heavy VAR loads, but the generator does. Heavy VAR loads will cause large voltage dips and overcurrent. Heavy watt loads will cause engine speed drop, overload, exhaust temperature rise and it simply may not recover and melt pistons in 2 minutes time.